On the evolution of thermally stratified layers at the top of Earth's core

Stable stratification at the top of the Earth's outer core has been suggested based upon seismic and geomagnetic observations, however, the origin of the layer is still unknown. In this paper we focus on a thermal origin for the layer and conduct a systematic study on the thermal evolution of the core. We develop a new numerical code to model the growth of thermally stable layers beneath the CMB, integrated into a thermodynamic model for the long term evolution of the core. We conduct a systematic study on plausible thermal histories using a range of core properties and, combining thickness and stratification strength constraints, investigate the limits upon the present day structure of the thermal layer. We find that whilst there are a number of scenarios for the history of the CMB heat flow, Qc, that give rise to thermal stratification, many of them are inconsistent with previously published exponential trends in Qc from mantle evolution models. Layers formed due to an exponentially decaying Qc are limited to 250–400 km thick and have maximum present-day Brunt-Väisälä periods, TBV = 8 − 24 hrs. When entrainment of the lowermost region of the layer is included in our model, the upper limit of the layer size is reduced and can fully inhibit the growth of any layer if our non-dimensional measure of entrainment, E > 0.2. The period TBV is insensitive to the evolution and so our estimates remain distinct from estimates arising from a chemical origin. Therefore, TBV should be able to discern between thermal and chemical mechanisms as improved seismic constraints are obtained.